Latent Heat of Fusion Definition
Heat energy required to change unit mass of a substance from solid to liquid state at its melting point without change in its temperature is called its latent heat of fusion.
When a substance is changed from solid to liquid state by adding heat, the process is called melting or fusion. The temperature at which a solid starts melting is called its fusion point or melting point.When a process is reversed i.e., when a liquid is cooled, it changes into solid state. The temperature at which a substance changes from liquid to solid state is called its freezing point. Different substances have different melting points. However, the freezing point of a substance is the same as its melting point.
It is denoted by Hf.
Hf = ΔQf/ m
or Δ Qf = m Hf … …. … … (4)
- Ice changes at 0° C into water. Latent heat of fusion of ice is 3.36 × 105 J kg -1. That is; 3.36 × 105 Joule heat is required to melt 1 kg of ice into water at 0°C.
Latent heat of vaporization
Heat energy required to change unit mass of a substance from liquid to gaseous state at its boiling point without change in its temperature is called latent heat of vaporization.It is represented by H v. Its S.I unit is J/kg. Its formula is given as:
Hv = ΔQv/m
When water is heated,it boils at 100 °C under standard pressure.Its temperature remains 100 °C until it is changed completely into steam.Its latent heat of vaporisation is 2260000 J/kg.That is one kilogram of water requires 2260000 Joule heat to change it completely into steam at its boiling point.
Table of latent heat of vaporisation of some substances
|Substance||Heat of vaporisation (Hv) in kJ/kg|